The present invention relates to Burst-Mode Trans-Impedance Amplifiers, which can have controlled- or uncontrolled-gain, for point-to-multipoint communication and fast optical switching applications.
Fiber-optic communication systems require an amplifier at the receiver to amplify the weak currents generated by the detector diode. These amplifiers must provide sufficient bandwidth, sensitivity, dynamic range, and output signal level to achieve good system performance. The most common amplifier in the fiber-optic field is known as a Trans-Impedance Amplifier (TIA). It is part of almost every optical transceiver. It consists of a high-gain amplifier and a feedback resistor.
Some new fiber-optic communication technologies, like point-to-multipoint links and switches, require fast signal “lock-in”, in addition to the common requirements of a TIA. In this case, the off-the-shelf components cannot meet the requirements. The reason for this is that it takes a relatively long period of time for a TIA to output a stable signal.
The majority of applications for a Burst-Mode TIA are in FTTH (Fiber-to-the-Home) networks in which a point-to-multipoint topology is used. For this application, we define burst mode to mean a transmission mode where data is transmitted in bursts rather than in continuous streams. In addition, fast optical switching applications require a Burst-Mode TIA in order to quickly output a reliable signal after switching has occurred.
A Burst-Mode optical receiver (or TIA) with fast response is required for Passive Optical Networks (PON) [see e.g. IEEE 802.3ah Draft Standard, p. 358, ITU-T Recommendation G.984.2 p. 27, and Maeda et al., IEEE Communications Magazine, vol. 40, p. 126-132, December 2001]. In PON systems, an optical line terminal (OLT) receives a burst of packet data with different optical powers due to point-to-multipoint communication. The receiver in the OLT must handle this type of packet data. The receiver requires high sensitivity, wide dynamic range, and quick response. Low cost and high reliability are also required in such PON networks.
Supporting the wide dynamic range is achieved by several existing methods. One of these methods utilizes high-speed Automatic Gain Control (AGC) [Yamashita et al., IEEE J. Solid-State Circuits, vol. 37, p. 881-886, July 2002; Le et al., ISSCC Dig. Tech. Papers, p. 474-475, February 2004]. The drawback of AGC is that it requires long acquisition time (hundreds of bits), making it unsuitable for applications that require fast acquisition.
Another approach utilizes DC cancellation from the input signal [Ota et al., IEEE J Lightwave Technol., Vol. 12, No. 2, p. 325-331, February 1994]. This may improve the dynamic range by a nominal amount (˜3 dB), but it also degrades the sensitivity of the receiver (1-3 dB degradation).
Another approach utilizes nonlinear gain [Nakamura et al., IEEE J Solid-State Circuits, vol. 33, p. 1179-1187, August 1998; Brigati et al., IEEE J. Solid-State Circuits, vol. 37, p. 887-894, July 2002]. This approach is hard to implement with silicon circuit fabrication technology, and degrades the sensitivity performance as well (1-2 dB degradation).
A further method utilizes programmable gain [Nakamura et al., ISSCC 2005, Optical Communication, Session 12.4]. This method involves selecting two or more gain values based on the input value. This method seems to be the best choice for the above-mentioned applications. It does not degrade the sensitivity performance and widens the dynamic range by a factor of approximately two (in dB) in the case of selecting between two gains. It is also fast (on the order of ten bits) and easy to implement.
There is thus a widely recognized need for, and it would be highly advantageous to have, a TIA that would have a wide dynamic range without sacrificing sensitivity performance, signal integrity, or response time. Furthermore, the need to provide these features and operate in burst-mode to accommodate multi-source packet data is finding an increasing number of industrial applications.
The present invention shows two different new architectures, using a programmable gain, that enable fast selection of the appropriate gain and keep the gain constant during a burst of data.